1. Field of the Invention
The present invention relates to isolated flyback power converters, and more particularly, to flyback converters employing primary side output voltage sensing.
2. Description of Related Art
Flyback power converters are well known in the art and provide the advantage of transformer isolation between the input and output sections, which is desirable for many applications. For example, many noise-sensitive devices require power supplies that are isolated from noisy primary supplies such as those presented by car batteries, avionics intermediate power busses, or industrial power supplies, among others. A simplified block diagram of a flyback converter, typical of the prior art, is depicted in FIG. 1. Its basic operation is summarized as follows.
On the primary side, switch 116 is closed, causing current supplied by the primary supply 102 to begin to flow through the primary windings 104 of the transformer 120. The rising current through the primary windings induces a voltage across the secondary windings 106 of the transformer that is roughly equal to n times the supply voltage Vin, where n is the transformer turns ratio. The polarity of the induced voltage is arranged such that it reverse biases diode 108, which prevents current from flowing. At a later time, switch 116 is opened, shutting off the primary current. In order to sustain the magnetic flux within the transformer 120, a large voltage is induced in the secondary winding 106 that forward biases diode 108. The resulting current charges the buffer capacitor 112, which sources the output current at a certain voltage Vout 110 to drive the load. The primary-side switch 116 is alternatively opened and closed to keep the output buffer capacitor 112 charged to the desired target voltage Vout. The precise value of Vout depends on the switching duty cycle of the primary switch 116. Therefore, a control loop is typically formed by sensing the output voltage using a sense circuit 118 to provide feedback to a pulse width modulator 114 that varies the switch duty cycle to control the output voltage.
However, because the secondary output 110 is electrically isolated from the primary source 102 via transformer 120, the sense circuit 118 must maintain that isolation. Many prior art systems therefore employ optical isolators, which have the advantage of maintaining a high degree of isolation but also bring the disadvantages of complexity, increased parts count and cost, and component aging issues that degrade performance over time. As an alternative to optical isolation, some prior art systems use auxiliary transformer windings to monitor the output voltage. Again, this introduces additional complexity and increases the parts count.
Therefore, it is desirable to monitor the output voltage from the primary circuit side to avoid the complexities of placing a sensor on the other side of the isolation barrier. Monitoring the output voltage from the primary side, a technique known as “primary side sensing,” has been discussed by some researchers in the field. For example, U.S. Pat. No. 7,463,497 to Negrete describes one method of primary side sensing.
In general, primary side sensing relies on the principle that the secondary output voltage Vout is reflected at the primary side. FIG. 2 illustrates a typical voltage waveform at the primary side switch 116 (see FIG. 1) of an operating flyback converter. Two cycles of the PWM waveform 202 are shown in FIG. 2. During the off cycle 212 of the primary side switch, the diode 108 remains in a conducting state during time interval 210 until the energy stored in the inductor has been delivered to the output. During the diode conduction interval, the voltage waveform 202 is equal to the sum of the input voltage Vin, indicated at 208, and n times the output voltage Vout, indicated at 206, plus the voltage dropped across the forward-biased diode 108, which decreases as the diode current drops until it shuts off, creating a knee 204 in the switch voltage waveform 202. Again, n refers to the transformer turns ratio. After the knee 204, the voltage waveform exhibits a damped oscillation until the primary switch 116 is switched on during the time interval indicated at 214. Therefore, by measuring the primary side voltage during the diode conduction interval 210, an indication of the output voltage Vout may be obtained.
The system discussed by Negrete in U.S. Pat. No. 7,463,497 uses a dual sample-and-hold circuit coupled to a control circuit such that each of the two sample and hold circuits capture alternating measurements of the PWM switch voltage during the diode conduction interval. The control circuit attempts to identify the first measurement taken that falls after the knee 204 and then backs up to the previous sampled value as an estimate of the output voltage. Such a system suffers from complexity in that two sample and hold circuits must operate at high speed to obtain multiple measurements of the switch voltage within the PWM cycle period, and the control circuit must be able to identify the knee in the voltage curve and be able properly select the correct sample instant for estimating the output voltage.
Accordingly, it would be useful to simplify the technique of obtaining an estimate of the output voltage to reduce reliance on multiple high-speed sample and hold circuits and to simplify the algorithm for identifying the knee 204 in the PWM voltage curve.